Enforce isalnum/isdigit receive ASCII characters

Fixes https://todo.sr.ht/~sircmpwn/scdoc/13

src/main.c

@@ -15,7 +15,7 @@ static int parse_section(struct parser *p) {
 	str_t *section = str_create();
 	uint32_t ch;
 	while ((ch = parser_getch(p)) != UTF8_INVALID) {
-		if (isdigit(ch)) {
+		if (ch < 0x80 && isdigit(ch)) {
 			assert(str_append_ch(section, ch) != -1);
 		} else if (ch == ')') {
 			if (!section->str) {
@@ -47,7 +47,7 @@ static void parse_preamble(struct parser *p) {
 	struct tm *now_tm = localtime(&now);
 	strftime(date, sizeof(date), "%F", now_tm);
 	while ((ch = parser_getch(p)) != UTF8_INVALID) {
-		if (isalnum(ch)) {
+		if (ch < 0x80 && isalnum(ch)) {
 			assert(str_append_ch(name, ch) != -1);
 		} else if (ch == '(') {
 			section = parse_section(p);